Stimulation of the immune system, which includes stimulation of either or both innate immunity and adaptive immunity, is a complex phenomenon that can result in either protective or adverse physiologic outcomes for the host. In recent years there has been increased interest in the mechanisms underlying innate immunity, which is believed to initiate and support adaptive immunity. This interest has been fueled in part by the recent discovery of a family of highly conserved pattern recognition receptor proteins known as Toll-like receptors (TLRs) believed to be involved in innate immunity as receptors for pathogen associated molecular patterns (PAMPs). Compositions and methods useful for modulating innate immunity are therefore of great interest, as they may affect therapeutic approaches to conditions involving autoimmunity, inflammation, allergy, asthma, graft rejection, graft versus host disease (GvHD), infection, cancer, and immunodeficiency.
Toll-like receptors (TLRs) are type I transmembrane proteins that allow organisms (including mammals) to detect microbes and initiate an innate immune response (Beutler, B., Nature 2004, 430:257-263). They contain homologous cytoplasmic domains and leucine-rich extracellular domains and typically form homodimers that sense extracellular (or internalized) signals and subsequently initiate a signal transduction cascade via adaptor molecules such as MyD88 (myeloid differentiation factor 88). There is such high homology in the cytoplasmic domains of the TLRs that, initially, it was suggested that similar signaling pathways exist for all TLRs (Re, F., Strominger, J. L., Immunobiology 2004, 209:191-198). Indeed, all TLRs can activate NF-kB and MAP kinases; however, the cytokine/chemokine release profiles derived from TLR activation appears unique to each TLR. Additionally, the signaling pathway that TLRs stimulate is very similar to the pathway that the cytokine receptor IL-1R induces. This may be due to the homology that these receptors share, i.e., TIR (Toll/IL-1R homology) domains. Once the TIR domain is activated in TLRs and MyD88 is recruited, activation of the IRAK family of serine/threonine kinases results which eventually promotes the degradation of Ik-B and activation of NF-kB (Means T. K., et al. Life Sci. 2000, 68:241-258). While it appears that this cascade is designed to allow extracellular stimuli to promote intracellular events, there is evidence that some TLRs migrate to endosomes where signaling can also be initiated. This process may allow for intimate contact with engulfed microbes and fits with the role that these receptors play in the innate immune response (Underhill, D. M., et al., Nature 1999, 401:811-815). This process might also allow host nucleic acids, released by damaged tissues (for example, in inflammatory disease) or apoptosis to trigger a response via endosomal presentation. Among mammals, there are 11 TLRs that coordinate this rapid response. A hypothesis put forward years ago (Janeway, C. A., Jr., Cold Spring Harb. Syrup. Quant. Biol. 1989, 54:1-13) that the innate immune response initiates the adaptive immune response through the pattern of TLR activation caused by microbes has now been substantiated. Thus, the pathogen-associated molecular patterns (PAMPs) presented by a diverse group of infectious organisms results in a innate immune response involving certain cytokines, chemokines and growth factors followed by a precise adaptive immune response tailored to the infectious pathogen via antigen presentation resulting in antibody production and cytotoxic T cell generation.
Gram-negative bacterial lipopolysaccharide (LPS) has long been appreciated as an adjuvant and immune-stimulant and as a pharmacological tool for inducing an inflammatory reaction in mammals similar to septic shock. Using a genetic approach, TLR4 was identified as the receptor for LPS. The discovery that LPS is an agonist of TLR4 illustrates the usefulness of TLR modulation for vaccine and human disease therapy (Aderem, A.; Ulevitch, R. J., Nature 2000, 406:782-787). It is now appreciated that various TLR agonists can activate B cells, neutrophils, mast cells, eosinophils, endothelial cells and several types of epithelia in addition to regulating proliferation and apoptosis of certain cell types.
To date, TLR7 and TLR8, which are somewhat similar, have been characterized as receptors for single-stranded RNA found in endosomal compartments and thus thought to be important for the immune response to viral challenge. Imiquimod, an approved topical antiviral/anti-cancer drug, has recently been described as a TLR7 agonist that has demonstrated clinical efficacy in certain skin disorders (Miller R. L., et al., Int. J. Immunopharm. 1999, 21:1-14). This small molecule drug has been described as a structural mimetic of ssRNA. TLR8 was first described in 2000 (Du, X., et al., European Cytokine Network 2000 (September), 11(3):362-371) and was rapidly ascribed to being involved with the innate immune response to viral infection (Miettinen, M., et al., Genes and Immunity 2001 (October), 2(6):349-355).
Recently it was reported that certain imidazoquinoline compounds having antiviral activity are ligands of TLR7 and TLR8 (Hemmi H., et al. (2002) Nat. Immunol. 3:196-200; Jurk M., et al. (2002) Nat. Immunol. 3:499). Imidazoquinolines are potent synthetic activators of immune cells with antiviral and antitumor properties. Using macrophages from wildtype and MyD88-deficient mice, Hemmi et al. recently reported that two imidazoquinolines, imiquimod and resiquimod (R848), induce tumor necrosis factor (TNF) and interleukin-12 (IL-12) and activate NF-icB only in wildtype cells, consistent with activation through a TLR (Hemmi H., et al. (2002) Nat. Immunol. 3:196-200). Macrophages from mice deficient in TLR7 but not other TLRs produced no detectable cytokines in response to these imidazoquinolines. In addition, the imidazoquinolines induced dose-dependent proliferation of splenic B cells and the activation of intracellular signaling cascades in cells from wildtype but not TLR7−/− mice. Luciferase analysis established that expression of human TLR7, but not TLR2 or TLR4, in human embryonic kidney cells results in NF-KB activation in response to resiquimod. The findings of Hemmi et al. thus suggest that these imidazoquinoline compounds are non-natural ligands of TLR7 that can induce signaling through TLR7. Recently it was reported that R848 is also a ligand for human TLR8 (Jurk M., et al. (2002) Nat. Immunol. 3:499).
In view of the great therapeutic potential for compounds that modulate toll-like receptors, and despite the work that has already been done, there is a substantial ongoing need to expand their use and therapeutic benefits.